Redundancy is generally used in order to minimize network down time. This may be in the form of a backup link when a primary link fails. It may also be in the form of a back up link card (e.g., line interface card) when a primary link card fails. This is referred to as card redundancy. In a one-to-one card redundancy, every card has a corresponding redundant card. One card acts as an active card and the other card acts as a standby card. The standby card may be a hot standby card. The hot standby card is powered and ready to take the role of the active card, if the existing active card fails.
The standby card is generally synchronized with the active card. Synchronization can take place continuously while the active card is still functional. For example, the active card may receive multiple add connection and delete connection commands and transfer these commands to the standby card. These connection commands are serviced by the active card using a pool of connection resources on the active card. These connection resources are referred to as LCN (logical channel number or logical connection number) resources. Similarly, these commands need to be replayed on the standby card in order to synchronize the standby card with the active card.
The standby card may be in sync with the active card if the standby card and the active card were powered on at a same time because the standby card would have received continuous updates sent from the active card. Alternatively, the standby card may be in sync with the active card if the standby card is powered on at a subsequent time from when the active card was powered on and if the active card does not have any connections.
However, when the active card already has multiple connections and the standby card is subsequently powered on, the standby card connection information is not the same as the connection information in the active card. This is because the LCN pool on the standby card is full (free) upon initialization. Since there is no history of events that occurred on the active card, the standby card cannot recreate the events in order to synchronize its LCN resources with the LCN resources on the active card. The active card can only transfer current state or the existing state to the standby card. This situation can lead to sparse array or ‘holes’ in the LCN pool of the standby card.
The standby card changes role from a standby role to an active role when the active card fails. Typically, when the active card fails, new connection provisioning, modification or deletion commands are lost and traffic loss occurs. Delay in the role change of the standby card from its standby role to the active role directly affects the amount of traffic loss.